1. Field of the Invention
This invention relates to an image reading apparatus. In particular, it relates to a flatbed scanner capable of reading a transparency. More particularly, it relates to a scanner having a light source which synchronously moves with the image sensor of the scanner.
2. Description of Prior Art
Flatbed scanners are generally used to scan an opaque sheet in a reflective manner. A lamp tube is used to illuminate the scanned object, and an optical system is used to collect and pass the reflected light to the image sensor. The light distributed on the scanned object is of the same intensity since the distance between the lamp tube and the scanned object is fixed while scanning. Further, since the distance between the image sensor and the scanned object is also fixed, the brightness of the image received by the image sensor is uniform along the scanning path. Regarding the variation of the gray level for the image along the direction of the lamp tube due to its uneven illumination, a correction sheet can be used to build a reference level of brightness before performing the scanning. Thus the signal detected by the image sensor for each pixel can be corrected while scanning so that the image sensor determines the amplitude of the signals based only on the variation of the gray level on the document and neglects the variation caused by the light source.
While the scanner scans a transparent sheet, a transparency kit is provided to provide a light source. The light emitted by the light source could pass through the transparent sheet and enters the optical system beneath the transparent sheet. Please refer to FIG. 1a and FIG. 1b. If the light emitted by the light source 12 is not uniform, the change of the relative position of the light source 12 and the image sensor 10 would make the light intensity detected by the image sensor 10 also changed. Therefore, the image sensor 10 and the light source 12 above the scanned document must move synchronously. If there is any discrepancy in the synchronous moving of the image sensor 10 and the light source 12, the scanning quality would deteriorate because the light intensity of the image distributed on the image sensor 10 varies, as shown in FIG. 1b. 
Please refer to FIG. 2. For a scanner capable of reading a transparency, the scanner generally includes motors 14, 24 together with belts 16, 26 to move the lamp tubes 18, 28 which serve as the light sources for scanning an opaque object and a transparency respectively. The motor 14 driving the image sensor 20 and the motor 24 driving the light source of the transparency kit are respectively controlled to synchronously move the image sensor 20 and the light source 28. However, the motors 14, 24 might not run smoothly, the two motors 14, 24 might not move exactly synchronously to each other, and the gears driving the belts 16, 26 might not be perfect circles. These factors would make the relative position of the light source 28 and the image sensor 20 unlikely to be fixed. Therefore, the conventional scanner cannot provide a stable and good scanning quality while scanning a transparency.
Accordingly, to improve the drawbacks of the prior-art scanner, the object of this invention is to provide a transparency scanner having a light source which synchronously moves with the image sensor of the scanner. Such arrangement would be capable of preventing deterioration of the scanning quality due to the variation of light intensity from the light source.
To attain the above object, this invention does not use different motors to respectively drive the transparency scanning light source and the image sensor as in the conventional scanner. Instead, this invention provides a strong link between the light source and the image sensor to provide a conjunctive relation, and uses only one motor to move the transparency scanning light source and the image sensor synchronously. Thereby, errors caused by the driving devices such as different motors or gears, etc., are avoided.
In the invention, the link between the transparency scanning light source and the image sensor is realized by magnetic force. However, in place of magnetic force, other ways can be used, e.g., by means of mechanical link. Once a strong connection is established between the transparency scanning light source and the image sensor, the relative positions of the transparency scanning light source and the image sensor remains are fixed while scanning. Thus the intensity distribution of light detected by the image sensor is constant during scanning, so that the scanning quality is not adversely affected.